Semiconductive Polyolefin Composition

ABSTRACT

A semiconductive polyolefin composition comprising (i) up to 80 wt. % of a polyolefin (I), (ii) carbon black, and (iii) up to 60 wt. % of a polymer (II) having a melting point of not less than 100° C., wherein the polyolefin (I) comprises two different ethylene copolymers: a first ethylene copolymer and second ethylene copolymer; wherein the first ethylene copolymer comprises polar group containing co-monomers in the amount of 3.5 mol % or less; and wherein the second ethylene copolymer comprises silane group containing co-monomers in the amount of 0.7 mol % or less.

The present invention relates to a semiconductive polyolefin compositionproviding superior mechanical and processing properties. The presentinvention further relates to a process for producing such asemiconductive polyolefin composition and its use in a semiconductivelayer of a power cable.

Usually, medium and high voltage power cables comprise at least a firstsemiconductive layer, an insulating layer and a second semiconductivelayer coated in that order on a conductor, e.g. made of aluminium orcopper. The first and second semiconductive layers can be made of thesame or different materials. Such semiconductive layers are normallycross-linkable so that they can be cross-linked or leftnon-cross-linked, as occasion demands.

Furthermore, the second semiconductive layer can be bonded to theinsulation layer or can be strippable from that layer. Such a strippablelayer can in principle be crosslinked or non-crosslinked.

The crosslinkable material in a semiconductive layer for a power cablehas conventionally been a low density copolymer of ethylene (LDPEcopolymer) produced in a high pressure polymerisation process.Conventionally, the crosslinking is effected after formation of thelayered cable structure. The crosslinking is usually performed by theperoxide technology based on radical formation or by using crosslinkablevinyl co-monomers containing functional groups which are introduced bycopolymerisation of ethylene with the co-monomer or by chemicalgrafting.

An example of crosslinking using functional groups is the so calledsilane technology, in which silane-group containing co-monomerscontained in the ethylene polymer are exposed to moisture upon whichsilanol groups are formed. The silanol groups are then crosslinked bycondensation in the presence of a silanol condensation catalyst.

If the peroxide technology is applied, care must be taken in theextrusion step of the polymer based layer materials because undesiredcrosslinking may occur during the extrusion step due to the elevatedtemperature used therein. On the other hand, the extrusion temperatureof the layer material needs to be kept above the melting point of thepolymer based layer materials and, usually, the difference between themelting point of the polymer and the extrusion temperature should be atleast 20 to 25° C. in order to achieve desirable homogeneity.

For practical reasons, the production window of LDPE based polymers hasbeen limited in prior art so that LDPE copolymers with a melting pointof below 95° C. have typically been used. As conventional LDPEhomopolymers typically have a melting point of at least 110° C., it hasnot been possible to use these homopolymers as such for semiconductivelayer applications.

Furthermore, it is highly desirable to have certain mechanicalproperties, such as flexibility (elasticity), in the polymeric materialsused for the production of the layers of a power cable. The cablesshould be flexible enough in order to withstand i.a. bending. Thus,crack and creep resistance of the layer materials are importantfeatures. The flexibility level of a cable is in practice defined by theinsulation layer material which must, however, meet the demands ofelectrical insulating properties required for power cables. Insimplified terms, the electrical insulating properties are prioritisedover flexibility properties in insulation materials. Thus in practice,the semiconductive layer of a power cable should have a flexibility ofat least the same level as that of the neighbouring insulating layer inorder to provide a cable product being free of fractures along thethickness of the layers.

LDPE as such is flexible, but the semiconducting layers comprisenormally carbon black (CB) to provide certain conductivity. However,carbon black further reduces the elastic properties of the polymermaterial. For that reason, co-monomers are usually incorporated inrather high amounts to increase the elasticity of LDPE based polymermaterials for the semiconductive compositions.

By incorporation of such co-monomers increasing the elasticity, thecrystallinity of the LDPE based polymer material is decreased.Therefore, according to conventional technologies, only LDPE basedpolymer materials having a crystallinity below 25% were used.

Moreover, the “low” melting point of the LDPE based polymer bringsdrawbacks also to the other processing steps in the preparation processof a semiconducting cable (especially medium voltage (MV) power cableapplications), e.g. to the crosslinking step of the extruded cablestructure. Namely, in the preparation process of power cables, the threelayers are conventionally co-extruded on the conductor and then thecables are subjected to a crosslinking step in elevated temperature,typically to a steam or water treatment in elevated temperature (wateris used particularly in case of silane-group containing crosslinkablematerial).

It is known that the crosslinking speed increases with the increase ofthe temperature. On the other hand, the melting point of the outersemiconductive layer material sets a limit to the usable temperatureincrease in the crosslinking step, since the layer softens and becomessticky, when the temperature becomes close to the melting temperature.As a result, the wound cable in a cable drum (or reel) in the water bathadheres to neighbouring wound cable surfaces. Additionally, thedeformation resistance of the cable is decreased. This is a problemespecially for cables, such as for MV cables, having a semiconductivelayer as the outer layer. Thus, the material of the outer semiconductivelayer limits the usable crosslinking temperature resulting in longercuring times, and hence the maximum temperature used during thecrosslinking step independent of the used crosslinking technology hasconventionally been between 75 to 80° C.

A strippable semiconductive resin composition is known from U.S. Pat.No. 6,284,374 B1 which is suitable for an outer semiconductive layer ofa crosslinked polyolefin-insulated wire and cable. Such a strippablesemiconductive resin composition comprises a polymer component with anumber average molecular weight of not less than 3×10⁴ or a weightaverage molecular weight of not less than 3×10⁵ and a melting point of60 to 80° C., composed mainly of an ethylene-vinyl acetate copolymer, ora polymer component (b) composed mainly of 99 to 50 wt.-% of theethylene-vinyl acetate copolymer and 1 to 50 wt.-% of a polyolefinhaving a melting point of 120° C. or above and conductive carbon blackcompounded with polymer component (a) or (b) to give a semiconductivelayer having a volume resistivity of not more than 5000 Ω×cm at roomtemperature. However, as the ethylene copolymer according to thisreference has an excessively low melting point and thus relatively highpolar comonomer content, whereby the crosslinking step is carried out inthe conventionally used temperature, see example 1, column 6, line 49.On the other hand, U.S. Pat. No. 6,525,119 B2 discloses a compositionuseful for an internal semiconductive layer in a power cable having goodadhesion qualities, low heat deformation and which can be extruded atcomparatively high temperatures. Such a semiconductive polymercomposition comprises one or more copolymers selected from the groupconsisting of (I), a copolymer of ethylene and vinyl acetate containingabout 10 to about 50 wt.-% vinyl acetate; (II) a copolymer of ethyleneand ethyl acrylate containing about 10 to about 50 wt.-% ethyl acrylate;(III) a copolymer of ethylene and butyl acrylate containing about 10 to50 wt.-% butyl acrylate and based upon 100 parts of component (a), (b)about 55 to about 200 parts of a linear copolymer of ethylene and analpha-olefin having 3 to 12 carbon atoms, (c) about 1 to about 50 partsof a specific organopolysiloxane, (d) about 10 to about 350 parts ofcarbon black and (e) optionally up to about 2 parts of an organicperoxide. Again in this embodiment, due to the excessively high contentof the co-monomer in the ethylene copolymer, the melting point isexcessively low so that the above-described drawbacks in thecrosslinking step apply and the processing speed must be sacrificed. Asimilar semiconductive resin composition for an insulation shield for amoisture-cured insulation layer comprising one or more copolymersselected from copolymers of ethylene and high amounts of a polarco-monomer are described in U.S. Pat. No. 6,706,791 B2 involving thesame drawbacks as described above.

Therefore, it is an object of the present invention to provide asemiconductive polymer composition with improved mechanical properties,including a decreased stickiness, and improved processing propertiesincluding expanded processing temperatures.

Moreover, it is an object of the present invention to provide such asemiconductive polymer composition with improved processing propertiessuch as homogeneity, cross-linkability and/or flexibility and elasticityof a final cable structure.

Unexpectedly and against the prior art teachings it has been found thatthese objects are achieved by providing a semiconductive polyolefincomposition which contains a polyolefin with a rather low amount ofco-monomer and thus with a rather high melting temperature.

The present invention therefore provides a semiconductive polyolefincomposition comprising:

-   -   (i) up to 80 wt. % of a polyolefin (I),    -   (ii) carbon black, and    -   (iii) optionally up to 60 wt. % of a polymer (II) having a        melting point of not less than 100° C.,        wherein the polyolefin (I) has a co-monomer content of equal to        or less than 4.3 mol %. The “comonomer content” of        polyolefin (I) means herein the total content of comonomer(s)        present in polyolefin (I) component.

The comonomer content of polyolefin (I) in mol % is based on the totalmolar amount of monomers in polyolefin (I).

Polymer (II) may also be a blend of different polymers, wherein theproperties defined herein for polymer (II) apply for the entire blendwhen applicable.

The inventive polymer composition ensures high elasticity andflexibility of a semiconductive cable layer comprising the compositionwith absence of crack and creep if wound on a cable drum.

According to the prior art, it was expected that such high meltingpolymers having low amounts of co-monomers as described above, would inturn sacrifice the processing properties, homogeneity and/or flexibilityof the prepared cable structure. Unexpectedly and surprisingly, thepresent invention provides a very good balance between mechanical andprocessing properties. Thus, the inventive semiconductive polyolefincomposition can be produced by a more simple production process andsemiconductive layer compositions may be provided, which show improvedresistance to heat deformation and decreased stickiness when wound on acable drum.

Furthermore, the material for such a semiconductive layer having theinventive semiconductive polymer composition expands the processingwindow for producing the semiconductive layer enabling highercrosslinking temperatures resulting in shorter curing times and higherprocessing speed.

According to the present invention it is preferred that the polyolefin(I) has a degree of crystallinity of not less than 27%, more preferablynot less than 29%, measured according to the methods as described below.

The upper limit of the crystallinity of polyolefin (I) is not criticaland, as evident for the skilled person, depends on the used material.

Preferably, the polyolefin (I) is contained in the inventive polymercomposition in the range of from 40 to 80 wt.-%, more preferably from 40to 60 wt. %, still more preferably from 45 to 60 wt. %, and mostpreferably from 55 to 60 wt.-%, based on the total weight of thesemiconductive polyolefin composition.

Preferably, polymer (II) is present in the composition, preferably in anamount of up to 60 wt.-%, more preferably in an amount of from 5 to 40wt.-%, still more preferably of from 10 to 35 wt.-%, even morepreferably of from 10 to 30 wt.-% based on the weight of the totalamount of polyolefin (I) and polymer (II), more preferably based on thetotal weight of the semiconductive polyolefin composition.

Furthermore, preferably, the weight ratio polyolefin (I):polymer (II) isfrom 5:1 to 1:1, more preferably from 4:1 to 1.5:1.

In a preferred embodiment of the present invention, in the compositionapart from polyolefin (I) and polymer (II) no further polymericcomponents are present, i.e. the entirety of the polymeric components ofthe composition, which preferably makes up at least 90 wt. % of thetotal composition, consists of polyolefin (I) and polymer (II).Preferably, the polymer (II) has a melting point of up to 170° C., morepreferably has a melting point in the range of from 100 to 170° C., andstill more preferably from 110 to 135° C., measured according to ISO3146.

It is further preferred that the polyolefin (I) is an ethylene homo- orcopolymer, more preferably is a high pressure ethylene copolymer (LDPEcopolymer) comprising one or more comonomers. The high pressure LDPEcopolymer (I) is produced by high pressure radical polymerisation. Highpressure polymerisation is a well known technology in the polymer fieldand can be effected in a tubular reactor or an autoclave reactor.Preferably, it is a tubular reactor. Further details about high pressureradical polymerisation are given in WO 93/08222.

In a high pressure process, the polymerisation is generally performed atpressures in the range of 1200 to 3500 bar and at temperatures in therange of 150 to 350° C.

The high-pressure polyethylene of component (I) preferably has a meltingpoint of not less than 95° C., more preferably not less than 97° C.,measured according to ASTM D3418. In one preferred embodiment, themelting point of the polyolefin (I) is as high as 98° C. or more, even100° C. or more, may be desired. The upper limit is not particularlylimited, and can be e.g. less than 115° C.

Preferably, the polyolefin (I) is a copolymer of ethylene with at leasta polar group containing comonomer which is not a silane groupcontaining polar comonomer.

The semiconductive polyolefin composition can be non-crosslinkable orcrosslinkable. Preferably said semiconductive polyolefin composition iscrosslinkable for producing a cable comprising crosslinkedsemiconductive layer(s).

In one embodiment the polyolefin (I) contains polar group containingcomonomer(s) as the comonomer only, whereby the polar comonomer contentis preferably 4.3 mol % or less, preferably 4.0 mol % or less, morepreferably 3.5 mol % or less and in some embodiments 2.5 mol % or lessor even 2.0 mol % or less may be desired.

In this embodiment, the MFR₂ (ISO 1133, 190° C., 2.16 kg) of polyolefin(I) is preferably 1 g/10 min or more, and is typically less than 50 g/10min. More preferably, the MFR2 is between 10 and 30 g/10 min. In case ofcrosslinkable semiconductive polyolefin composition the polyolefin (I)is preferably a terpolymer of ethylene with a polar group containingco-monomer and a silane group containing co-monomer. Thus, in anotherembodiment the polyolefin (I) is a terpolymer which may contain 4.3 mol% or less, preferably 4.0 mol % or less, more preferably 3.5 mol % orless, and in some embodiments even 2.5 mol % or less or 2.0 mol % orless of a polar group containing comonomer; and up to 1.0 mol %,preferably 0.5 mol % or less and in some embodiments even 0.4 or lessmol % of a silane group containing comonomer. The total comonomercontent of the polyolefin (I), i.e. terpolymer, is 4.3 mol % or less,preferably 3.5 mol % or less, more preferably 2.5 mol % or less. In someembodiments even 2.3 mol % or less may be desired. In case thepolyolefin (I) is a terpolymer as defined above or below, the MFR₂ (ISO1133, 190° C., 2.16 kg) of polyolefin (I) is preferably more than 0.01g/10 min, preferably is 0.1 to 15 g/10 min, and most preferably is 0.3to 10 g/10 min.

Polyolefin (I) may comprise one or more copolymers of olefins,preferably ethylene, with one or more comonomers, e.g. two differentcopolymers, such as two terpolymers. Then, the comonomer content ofpolyolefin (I) is the sum of comonomers present in each copolymer ofpolyolefin (I) and is less than 4.3% by mole (mol %), preferably equalto or less than 4.0 mol %, more preferably equal to or less than 3.5 mol%. In some embodiments equal to or less than 2.5 mol % may even bedesired. Preferably the polyolefin (I) consists of only one copolymer ofethylene with one or more comonomers as defined above.

In a further preferred embodiment, polyolefin (I) consists of aterpolymer which has a content of polar group containing comonomer of4.3 mol % or less, preferably 4.0 mol % or less, more preferably 3.5 mol% or less, most preferably 2.5 mol % or less and in some embodimentseven 2.0 mol % or less may be desired, and a content of silane groupcontaining comonomer of 0.7 mol % or less, preferably 0.5 mol % or less,more preferably 0.4 mol % or less, and in some embodiments even 0.38 mol% or less may be desired; provided that the total comonomer content ofpolyolefin (I) is 4.3 mol % or less, preferably 3.5 mol % or less, morepreferably 2.5 mol % or less. In some embodiments even 2.3 mol % or lessmay be desired. In all embodiments of polyolefin (I), the comonomercontent is usually 0.5 mol % or higher, preferably is 1.0 mol % orhigher and most preferably is 1.5 mol % or higher.

The density of polyolefin (I) in all embodiments is typically more than900 kg/m³, preferably 910 to 930 kg/m³, and most preferably 910 to 925kg/m³.

In the embodiments when silane group containing comonomers are presentin polyolefin (I) which may either be introduced by copolymerisation orgrafting, the amount of those is usually 0.1 mol % or higher, preferably0.2 mol % or higher.

The expression “polar group containing co-monomer” is intended to coverboth the cases where only one type of polar groups is present and thecase where two or more different types of polar groups are present.Similarly, the expression “silane group containing co-monomer” isintended to cover both the cases where only one type of silane group ispresent and the case where two or more different types of silane groupsare present.

Preferably, polar groups are selected from siloxane, amide, anhydride,carboxylic, carbonyl, hydroxyl, ester and epoxy groups.

The polar groups may be introduced into the polyolefin (I) bycopolymerisation of olefinic, including ethylene, monomers withco-monomers bearing polar groups.

Alternatively, the polar groups may also be introduced into the polymerby grafting of an ethylene polymer with a polar group containingcompound, i.e. by chemical modification of the polyolefin, by additionof a polar group containing compound mostly in a radical reaction.Grafting is e.g. described in U.S. Pat. No. 3,646,155 and U.S. Pat. No.4,117,195.

Preferred means for introducing the polar group is the copolymerisationwith polar comonomers.

As examples of comonomers having polar groups may be mentioned thefollowing: (a) vinyl carboxylate esters, such as vinyl acetate and vinylpivalate, (b) (meth)acrylates, such as methyl(meth)acrylate,ethyl(meth)acrylate, butyl(meth)acrylate and hydroxyethyl(meth)acrylate,(c) olefinically unsaturated carboxylic acids, such as (meth)acrylicacid, maleic acid and fumaric acid, (d) (meth)acrylic acid derivatives,such as (meth)acrylonitrile and (meth)acrylic amide, and (e) vinylethers, such as vinyl methyl ether and vinyl phenyl ether.

Amongst these comonomers, vinyl esters of monocarboxylic acids having 1to 4 carbon atoms, such as vinyl acetate, and (meth)acrylates ofalcohols having 1 to 4 carbon atoms, such as methyl(meth)acrylate, arepreferred. Especially preferred comonomers are butyl acrylate, ethylacrylate and methyl acrylate. Two or more such olefinically unsaturatedcompounds may be used in combination. The term “(meth)acrylic acid” isintended to embrace both acrylic acid and methacrylic acid.

Preferably, the polar group containing comonomer units are selected fromthe group of acrylates.

As mentioned the semiconductive polyolefin composition is preferablycrosslinkable, whereby the polyolefin (I) preferably comprisessilane-group containing monomer units. The silane groups may preferablybe introduced into a polyolefin (I) via copolymerisation of silanegroups containing comonomers with ethylene and optionally with othercomonomers, preferably with polar group containing comonomer(s), to forma terpolymer (I) as defined above.

Alternatively, the silane groups may also be incorporated to apolyolefin (I) via grafting, as e.g. described in U.S. Pat. No.3,646,155 and U.S. Pat. No. 4,117,195. For grafting, the same silanegroup-containing compounds may be used as are used as comonomers in thecase of copolymerisation.

In an especially preferred embodiment polyolefin (I) is an ethylenecopolymer which contains only polar comonomer units and silanegroup-containing comonomer units, which may have been introduced intothe polyethylene either by copolymerisation or by grafting.

If copolymerisation is used for introducing silane-group containingcomonomers into polyolefin (I), preferably the copolymerisation iscarried out with an unsaturated silane compound represented by theformula

R¹SiR² _(q)Y_(3−q)   (I)

wherein

R¹ is an ethylenically unsaturated hydrocarbyl, hydrocarbyloxy or(meth)acryloxy hydrocarbyl group,

R² is an aliphatic saturated hydrocarbyl group,

Y which may be the same or different, is a hydrolysable organic groupand q is 0, 1 or 2.

Special examples of the unsaturated silane compound are those wherein R¹is vinyl, allyl, isopropenyl, butenyl, cyclohexanyl orgamma-(meth)acryloxy propyl; Y is alkoxy, such as methoxy or ethoxy,alkylcarbonyloxy, such as formyloxy, acetoxy or propionyloxy, or analkyl- or arylamino group; and R², if present, is a methyl, ethyl,propyl, decyl or phenyl group.

A preferred unsaturated silane compound is represented by the formula

CH₂═CHSi(OA)₃   (II)

wherein A is a hydrocarbyl group having 1-8 carbon atoms, preferably 1-4carbon atoms.

Preferably, the silane group containing monomer units are selected fromthe group of vinyl tri-alkoxy silanes.

The most preferred compounds are vinyl trimethoxysilane, vinylbismethoxyethoxysilane, vinyl triethoxysilane,gamma-(meth)acryl-oxypropyltrimethoxysilane,gamma(meth)acryloxypropyltriethoxysilane, and vinyl triacetoxysilane.

The copolymerisation of the ethylene with polar group containingcomonomer and optionally with the unsaturated silane group containingcomonomer may be carried out under any suitable conditions resulting inthe copolymerisation of the monomers.

The optional polymer (II) may be any type of polymer suitably selectedfor the purposes of the present invention, as long as it possesses amelting point of not less than 100° C., preferably 100 to 170° C., morepreferably up to 165° C., such as 100 to 150° C. The polymer (II) may beselected from the group consisting of C₂ to C₄ polyolefins, polystyrene,and any blend thereof. It may especially comprise polyethylene, such asLDPE homo- or copolymer produced by high pressure polymerisation, a wellknown polyethylene produced by low pressure polymerisation, such aslinear low density polyethylene (LLDPE), medium density polyethylene(MDPE) or high density polyethylene (HDPE), polypropylene orpolybutylene. If the polymer (II) comprises or consists of polyethylene,this is preferably LDPE homopolymer.

When the polymer (II) is LLDPE or LDPE, preferably LDPE, the MFR₂ (ISO1133, 190° C., 2.16 kg) is preferably 1 g/10 min or more, and istypically less than 50 g/10 min. More preferably, the MFR₂ is between 10and 30 g/10 min. The density of polymer (II) in this embodiment istypically more than 900 kg/m³, preferably 910 to 930 kg/m³, and mostpreferably 910 to 925 kg/m³.

It is particularly preferred that the polymer (II) comprises or consistsof homopolymers of propylene, random copolymers of propylene orheterophasic copolymers of propylene.

In this embodiment, the MFR₂ (ISO 1133, 230° C., 2.16 kg) of polymer(II) preferably is more than 0.5 g/10 min and typically up to 100 g/10min, more preferably is between 1 and 30 g/10 min.

Suitable homo, random and heterophasic polypropylenes (PP) as component(II) may be any known PP polymer, e.g. any commercially available PPpolymer, or such PP polymer may be produced using common polymerisationprocesses known in the art. The polypropylene polymer may be produced ina single- or multistage process of propylene or propylene and one ormore comonomers, such as ethylene or higher alpha olefin. Thepolymerisation can be effected as bulk polymerisation, gas phasepolymerisation, slurry polymerisation, solution polymerisation orcombinations thereof using conventional catalysts includingZiegler-Natta and metallocene catalysts. E.g. homopolymer or randomcopolymer of polypropylene can be made either in one or two or more loopreactors or gas phase reactors or in a combination of a loop and atleast one gas phase reactor. In case of heterophasic copolymer ofpolypropylene the matrix of homo or random copolymer can be producede.g. in a single stage or as a multistage process described above andthe elastomeric (rubber) part of the propylene copolymer can be producedas a in-situ polymerisation e.g. in a separate reactor, e.g. gas phasereactor in the presence of the matrix polymer produced in the previousstage. Alternatively the elastomeric copolymer of propylene part can becompounded to the matrix phase material. The heterophasic copolymer ofpolypropylene thus is comprised of a matrix phase of homo- or copolymerof polypropylene and an elastomeric propylene copolymer dispersed in thematrix. Such heterophasic compounds e.g. are described in EP 1 244 717.The above processes are well known in the field. Preferably, saidpolypropylene (II) has a melting point of up to 165° C., such as 100 to150° C., preferably 100 to 135° C. Generally, suitable polymers aspolyolefin components (I) and (II) may be any known polymers, e.g.commercially available polymers, or they can prepared according to oranalogously to polymerisation methods described in the literature ofpolymer field. In a preferred embodiment of the polyolefin compositionof the present invention comprises polyolefin (I), polymer (II) andcarbon black. Furthermore, the composition preferably consists ofpolyolefin (I), polymer (II) and carbon black and usual additives in anamount of 5 wt. % or less, more preferably of 2 wt. % or less. Theamount of carbon black is selected so as to make the layer(s) of theinvention semiconducting. As the carbon black component, any carbonblack suitable for semiconductive layer can be chosen depending on thedesired use and conductivity of the composition. The amount of carbonblack can vary. Especially the amount of carbon black depend on theamounts of components (I) and (II) in the polyolefin composition.

Preferably, the crosslinking polymer composition comprises 15 to 50 wt %of carbon black, based on the weight of the semiconductive polyolefincomposition. In other preferred embodiments, the amount of carbon blackmay be 10 to 45 wt.-%, 20 to 45 wt %, 30 to 45 wt %, 35 to 45 wt % or 36to 41 wt %, based on the weight of the semiconductive polyolefincomposition.

As already mentioned carbon black can be used which is electricallyconductive, e.g. carbon blacks grades described with ASTM Nxxx codes,acetylene black, furnace black and Kjeten black. Examples of suitablecarbon blacks are disclosed e.g. in WO 98/014516.

Preferably, carbon black may be contained in an amount of up to 45wt.-%, based on the total weight of the composition. In this manner, thevolume resistivity of the inventive polyolefin composition canadvantageously be adjusted to 100000 Ω-cm or below, more preferably to1000 Ω-cm or below, or in some applications even to 100 Ω-cm or below,as determined according to ASTM D 991 and/or ISO 3915.

Volume resistivity is in a reciprocal relationship to electricalconductivity, i.e. the lower resistivity, the higher is conductivity.

The semiconductive polyolefin composition for one or more semiconductivelayers of a power cable of the present invention may or may not becrosslinkable, depending on the desired end use, as well known in thefield. In case of crosslinkable semiconductive polyolefin compositionthe crosslinking is effected after the formation (extrusion) of thelayered cable structure.

Crosslinking might be, achieved by all processes known in the art, inparticular by incorporating a radical initiator into the polymercomposition which after extrusion is decomposed by heating, thuseffecting crosslinking. Or, e.g. in case the semiconductive polyolefincomposition comprises a polyolefin (I) which contains silanol groups,preferably originating from the silanol containing comonomer, byincorporating a silanol condensation catalyst, which after production ofthe cable upon intrusion of moisture into the cable links together thehydrolysed silane groups, as well known in the field.

Examples for acidic silanol condensation catalysts comprise Lewis acids,inorganic acids such as sulphuric acid and hydrochloric acid, andorganic acids such as citric acid, stearic acid, acetric acid, sulphonicacid and alkanoic acids as dodecanoic acid.

Preferred examples for a silanol condensation catalyst are sulphonicacid and tin organic compounds.

Preferably, a Brönsted acid, i.e. a substance which acts as a protondonor, or a precursor thereof, is used as a silanol condensationcatalyst. Such Brönsted acids may comprise inorganic acids such assulphuric acid and hydrochloric acid, and organic acids such as citricacid, stearic acid, acetic acid, sulphonic acid and alkanoic acids asdodecanoic acid, or a precursor of any of the compounds mentioned.

Preferably, the Brönsted acid is a sulphonic acid, more preferably anorganic sulphonic acid.

Still more preferably, the Brönsted acid is an organic sulphonic acidcomprising 10 C-atoms or more, more preferably 12 C-atoms or more, andmost preferably 14 C-atoms or more, the sulphonic acid furthercomprising at least one aromatic group which may e.g. be a benzene,naphthalene, phenantrene or anthracene group. In the organic sulphonicacid, one, two or more sulphonic acid groups may be present, and thesulphonic acid group(s) may either be attached to a non-aromatic, orpreferably to an aromatic group, of the organic sulphonic acid.

Further preferred, the aromatic organic sulphonic acid comprises thestructural element:

Ar(SO₃H)_(x)   (III)

with Ar being an aryl group which may be substituted or non-substituted,and x being at least 1, preferably being 1 to 4.

The organic aromatic sulphonic acid silanol condensation catalyst maycomprise the structural unit according to formula (III) one or severaltimes, e.g. two or three times. For example, two structural unitsaccording to formula (III) may be linked to each other via a bridginggroup such as an alkylene group.

The currently most preferred compounds are dodecyl benzene sulphonicacid and tetrapropyl benzene sulphonic acid.

The silanol condensation catalyst may also be precursor of the sulphonicacid compound, for example the acid anhydride of a sulphonic acidcompound, or a sulphonic acid that has been provided with a hydrolysableprotective group, as e.g. an acetyl group, which can be removed byhydrolysis.

Furthermore, preferred sulphonic acid catalysts are those as describedin EP 1 309 631 and EP 1 309 632, namely

-   -   a) a compound selected from the group of    -   (i) an alkylated naphthalene monosulfonic acid substituted with        1 to 4 alkyl groups wherein each alkyl group is a linear or        branched alkyl with 5 to 20 carbons with each alkyl group being        the same or different and wherein the total number of carbons in        the alkyl groups is in the range of 20 to 80 carbons;    -   (ii) an arylalkyl sulfonic acid wherein the aryl is phenyl or        naphthyl and is substituted with 1 to 4 alkyl groups wherein        each alkyl group is a linear or branched alkyl with 5 to 20        carbons with each alkyl group being the same or different and        wherein the total number of carbons in the alkyl groups is in        the range of 12 to 80;    -   (iii) a derivative of (i) or (ii) selected from the group        consisting of an anhydride, an ester, an acetylate, an epoxy        blocked ester and an amine salt thereof which is hydrolysable to        the corresponding alkyl naphthalene monosulfonic acid or the        arylalkyl sulfonic acid;    -   (iv) a metal salt of (i) or (ii) wherein the metal ion is        selected from the group consisting of copper, aluminium, tin and        zinc; and    -   b) a compound selected from the group of        -   (i) an alkylated aryl disulfonic acid selected from the            group consisting of the structure:

and the structure:

wherein each of R¹ and R² is the same or different and is a linear orbranched alkyl group with 6 to 16 carbons, y is 0 to 3, z is 0 to 3 withthe provisio that y+z is 1 to 4, n is 0 to 3, X is a divalent moietyselected from the group consisting of —C(R₃)(R₄)—, wherein each of R₃and R₄ is H or independently a linear or branched alkyl group of 1 to 4carbons and n is 1; —C(═O)—, wherein n is 1; —S—, wherein n is 1 to 3and —S(O)₂—, wherein n is 1; and

-   -   -   (ii) a derivative of (i) selected from the group consisting            of the anhydrides, esters, epoxy blocked sulfonic acid            esters, acetylates, and amine salts thereof which is a            hydrolysable to the alkylated aryl disulfonic acid,            together with all preferred embodiments of those sulphonic            acids as described in the above-mentioned European patents.

Accordingly, crosslinking may be achieved by incorporating thecrosslinking agent, which may be a radical initiator such as azocomponent or preferably a peroxide, or the above described silanolcondensation catalyst into the polymer composition used for theproduction of one or more of the cable layers in amounts and mannerconventionally known in the field. Preferably, e.g. the silanolcondensation catalyst is incorporated to one or more of the layercompositions in an amount of 0.0001 to 6 wt.-%, more preferably of 0.001to 2 wt.-% and most preferably of 0.05 to 1 wt.-%.

The present invention further provides a power cable comprising at leastone semiconductive layer comprising the polyolefin composition of thepresent invention.

Preferably, the power cable may comprise a conductor, a firstsemiconductive layer (a), an insulation layer (b) and a secondsemiconductive layer (c), each coated on the conductor in this order,wherein at least one of the first and second semiconductive layer(s) (a;c) comprises a semiconductive polyolefin composition according to thepresent invention as described above.

In a further preferred embodiment of the inventive power cable both thefirst (a) and second (c) semiconductive layers, comprise, morepreferably consist of, the semiconductive polyolefin compositionaccording to the present invention.

In a further preferable embodiment, at least one of the first and secondsemiconductive layers (a; c) is crosslinkable, preferably both first (a)and second (c) semiconductive layers are crosslinkable.

According to another embodiment of the inventive power cable the secondsemiconductive layer (c) may be strippable or non-strippable, preferablynon-strippable. If strippable, then it may preferably be crosslinkable.

The insulation layer (b) is well known in power cable field and cancomprise any polymeric material suitable and/or conventionally used forsuch insulation layer. Also the insulation layer (b) is preferablycrosslinkable.

Accordingly, the invention also provides a process for producing a powercable, wherein the process comprises providing the semiconductivepolyolefin composition according to the present invention by blendingpolyolefin (I), carbon black, and optionally, polymer (II) together,optionally with additives, above melting point of the polymercomponents, extruding the melt at elevated temperature above the meltingpoint of the polymer components on a conductor for forming at least onesemiconductive polymer layer on a conductor for a power cable.

Preferably, the semiconductive polyolefin composition is co-extruded onthe conductor together with one or more further cable layer(s) formingpolymeric composition(s), thus providing a multilayered power cable,preferably a multilayered power cable defined above. After providing thelayered power cable structure, preferably the multilayered power cableas defined above, the cable is collected, and preferably wound onto acable drum.

The additives can be any conventional additive and in amounts used inthe field of semiconductive layers, such as stabiliser, preferably aphenolic stabiliser. The blending of the polyolefin (I), carbon blackand polymer (II) optionally with conventional additives is typicallycarried out in an extruder in a known manner, whereby such mixing stepprecedes the following co-extrusion step for forming the cable layers.

Preferably, the obtained power cable is a crosslinkable power cable,comprising at least a first semiconductive layer which comprises,preferably consists of, a crosslinkable semiconductive polyolefincomposition of the invention as defined above. More preferably, theobtained cable is a crosslinkable multilayered power cable as definedabove, wherein at least one of the first semiconductive layer (a), theinsulation layer (b) and a second semiconductive layer (c), preferablyall of (a), (b) and (c), are crosslinkable.

In case of a crosslinkable power cable, after the formation of thelayered cable structure, the power cable, preferably in cable drums isthen subjected to a crosslinking step in a manner known in the cablefield. The crosslinking is typically effected in a steam or watertreatment at elevated temperature. It is preferred that the crosslinkingtemperature is in the range of from 80 to 100° C., more preferably 85 to95° C.

The power cable of the present invention may be prepared according toconventional cable preparation methods which are per se known in theart.

Furthermore, the invention provides a non-crosslinked or, preferably, acrosslinked power cable. Further preferably, the cable is a multilayeredcable as defined above, wherein at least one of the layers of the powercable as described above is crosslinked. In a further preferredembodiment of the inventive power cable both the first (a) and second(c) semiconductive layers, comprise the semiconductive polyolefincomposition according to the present invention. Also preferably, theinsulation layer (b) is crosslinked.

The power cables of the present invention are especially suited formedium voltage (MV) and high voltage (HV) power cable applications.Preferably, the power cable of the invention is a MV power cable ratedbetween 3.3 to 36 kV.

The present invention is now described in more detail with reference tothe following non-limiting examples which serve to illustrate theadvantages and superiority of the present invention and shall not beinterpreted to limit its scope.

METHODS AND EXAMPLES

1. Methods

a) Melt Flow Rate

The melt flow rate is equivalent to the term “melt index” and isdetermined in accordance with ISO 1133 for polyethylenes at 190° C. andat loads 2.16 kg (MFR₂), at 5 kg (MFR₅) or at 21.6 kg (MFR₂₁). MFR₂ forpolypropylenes is measured (ISO 1133) at 230° C. at 2.16 kg. Melt flowrate values are indicated in g/10 min.

b) Stickiness

Stickiness was measured according to the following method:

-   -   1 mm plaques (10×10 cm) were made from pellets by pressing 10        min at 170° C.,    -   two plaques of same material were put together in an oven, and        on the top a 200 g weight was put,    -   the samples were stored in oven at testing temperature for 16 h,    -   the samples were taken out and conditioned at room temperature        for minimum 1 h,    -   it was tried by hand to separate the two plaques from each        other.

The stickiness behaviour was evaluated according to the following scale:

1: Not possible to separate, 3: Acceptable, 5: No adhesion.

c) Density

The density of the used polymers was measured in accordance with ASTMD792.

d) Volume Resistivity

The volume resistivity of the semiconductive material is measured oncrosslinked polyethylene cables according to ISO 3915 (1981).

Cable specimens having a length of 13.5 cm are conditioned at 1 atm and60±2° C. for 5±0.5 hours before measurement. The resistance of the outersemiconductive layer is measured using a four-terminal system usingmetal wires pressed against the semiconductive layer. To measure theresistance of the inner semiconductive layer, it is necessary to cut thecable in two halves, removing the metallic conductor. The resistancebetween the conductive silver paste applied onto the specimen ends isthen used to determine the volume resistivity of the innersemiconductive layer. The measurements were carried out at roomtemperature and 90° C.

The same procedure is used to determine the volume resistivity ofcompositions that have not yet been crosslinked.

e) Oil Adsorption Number

Oil adsorption numbers of the carbon black samples was measured inaccordance with ASTMD2414.

f) Iodine Number

Iodine numbers of the carbon black samples was measured in accordancewith ASTM D1510.

g) Melting Temperature, Crystallization Temperature (Tcr), and Degree ofCrystallinity (ASTM D3418):

The melting temperature Tm of the used polymers was measured inaccordance with ASTM D3418.

Tm and Tcr were measured with Mettler TA820 differential scanningcalorimetry (DSC) on 3±0.5 mg samples. Both crystallization and meltingcurves were obtained during 10° C./min cooling and heating scans between−10 to 200° C. Melting and crystallization temperatures were taken asthe peaks of endotherms and exotherms.

The degree of crystallinity was calculated by comparison with heat offusion of a perfectly crystalline polyethylene, i.e. 290 J/g.

h) Comonomer Content

Comonomer content (wt %) was determined in a known manner based onFourier transform infrared spectroscopy (FTIR) determination calibratedwith ¹³C-NMR. All FTIR methods were run by FTIR a Perkin Elmer 2000, 1scan, resolution 4 cm⁻¹. The peak for the comonomer was compared to thepeak of polyethylene (e.g. the peak for butyl acrylate at 3450 cm⁻¹ wascompared to the peak of polyethylene at 2020 cm⁻¹ and the peak forsilane at 945 was compared to the peak of polyethylene at 2665 cm⁻¹. Thecalibration with ¹³C-NMR is effected in a conventional manner which iswell documented in the literature. Such calibrations are evident for askilled person. As a reference for calibration, reference is made toHaslam J, Willis H A, Squirrel D C., “Identification and analysis ofplastics”, 2^(nd) Edition, London, Iliffe Books, 1972. The weight-% wasconverted to mol-% by calculation.

The polar comonomer content can also be analyzed by NMR, which givescorresponding results as Comonomer Content (NMR). The comonomer contentwas determined by using ¹³C-NMR. The ¹³C-NMR spectra were recorded onBruker 400 MHz spectrometer at 130° C. from samples dissolved in1,2,4-trichlorobenzene/benzene-d6 (90/10 w/w).

An alternative method to determine comonomer content (e.g. silane andpolar comonomer) is to use NMR-method which would give equal results tothe above X-ray and FTIR method, i.e. results would be comparable to thepurposes of the invention.

2. Compositions Prepared

The following formulations for semiconductive polyolefin compositionswere prepared separately and the components were mixed together in a 46mm BUSS co-mixer at the melt temperature of 165 to 200° C. in mannerwell known for a skilled person. The ingredients and components arelisted in Table 1 and the composition of the ingredients are listed inTable 2.

Preparation Examples

Polyolefin (I) of the Reference Semiconductive Composition Terpolymer 1(Reference)

Terpolymer 1 was produced by high-pressure polymerisation with freeradical initiation, wherein ethylene monomers were reacted with vinyltrimethoxysilane (VTMS) and butylacrylate (BA) co-monomers amounts so asto yield 1.8 wt % (0.36 mol %) silane content and 17 wt % (4.4 mol %) BAcontent in final terpolymer 1, under the action of a radical initiatorin a reactor at a high pressure of about 120 to 350 MPa and an elevatedtemperature of between 150 to 350° C. When the reaction was completed,the temperature and the pressure were lowered, and the resultingunsaturated polymer recovered.

Polyolefin (I) of the Semiconductive Composition of the Invention:

Terpolymer 2

Terpolymer 2 was produced as described for terpolymer 1, but adapting inthe known manner the (co-)monomer feeds to obtain final terpolymer 2with silane content of 2.0 wt % (0.37 mol %) and BA content of 8 wt %(1.9 mol %).

Polymer (II):

LDPE homopolymer was a commercially available grade obtainable fromBorealis and having a MFR² of 2 g/10 min, a density of 923 kg/m³ and amelting temperature of 111° C.

PP terpolymer was a commercially available C₂/C₄ terpolymer of propylenehaving a trade name of Borseal™ TD218BF, having a MFR₂ of 6.0 g/10 minand a melting temperature of 131° C.

Carbon Black

Carbon black was a commercial product with surface area between 30 to 80m²/g (measured by BET, nitrogen absorption). The other properties aregiven in Table 1.

TABLE 1 Polymers Density Melting point MFR₂ ASTM D792 ASTM D3418 g/10min kg/m³ ° C. Terpolymer1 4.5 920 94 Terpolymer2 0.6 921 97 LDPE 2 923111 PP 6 ( 230° C.) 131 Carbon Black Oil adsorp. nr. (ml/100 g) Iodinenr. (mg/g), ASTM D2414 ASTM D1510 170-200 80 Stabilizer: Phenolic typestabiliser

Compositions A to E were prepared as indicated in Table 2, in which alsothe properties of the compositions are given.

TABLE 2 A (ref) B C D E (Ref 2) Formulations Terpolymer1/wt. % 47.2554.25 Terpolymer2/wt. % 47.25 51.25 54.25 LDPE/wt. % 24 24 24 PP/wt. %15 15 Carbon black/wt. % 28 28 30 30 Carbon black/wt. % 24Stabiliser/wt. % 0.75 0.75 0.75 0.75 0.75 Properties Volume resistivity/<1000 <1000 <1000 <1000 <1000 Ohm cm ISO 3915 MFR₂₁ (190° C., 32 8.6 235 17 21.6 kg), ISO 1133 Stickiness 75° C. 3 5 4 5 3 80° C. 1 5 4 5 2 85°C. 1 4 4 4 1 90° C. 1 4 4 4 1

We claim:
 1. A semiconductive polyolefin composition comprising: (i) up to 80 wt. % of a polyolefin (I), (ii) carbon black, and (iii) up to 60 wt. % of a polymer (II) having a melting point of not less than 100 wherein the polyolefin (I) comprises a first ethylene copolymer and second ethylene copolymer; wherein the first ethylene copolymer comprises a polar group containing co-monomers in an amount of 3.5 mol % or less; and wherein the second ethylene copolymer comprises a silane group containing co-monomers in an amount of 0.7 mol % or less.
 2. The semiconductive polyolefin composition according to claim 1, wherein the polyolefin (I) has a degree of crystallinity of not less than 27%.
 3. The semiconductive polyolefin composition according to claim 1, wherein polyolefin (I) is present in an amount of 40 to 60 wt. %.
 4. The semiconductive polyolefin composition according to claim 1 wherein polymer (II) is present in an amount of 5 to 40 wt. %.
 5. The semiconductive polyolefin composition according to claim 1, wherein polymer (II) has a melting point in the range of from 100 to 170° C or from 110 to 135° C.
 6. The semiconductive polyolefin composition according to claim 1, wherein the polyolefin (I) is a high pressure low density ethylene (LDPE) copolymer.
 7. The semiconductive polyolefin composition according to claim 1, wherein the polyolefin (I) is a terpolymer of ethylene with a polar group containing co-monomer and a silane-group containing co-monomer.
 8. The semiconductive polyolefin composition according to claim 7, wherein the polar co-monomer is selected from the group consisting of acrylate and acetate co-monomers.
 9. The semiconductive polyolefin composition according to claim 7, wherein the silane-group containing co-monomer is selected from the group consisting of vinyl tri-alkoxy silanes.
 10. The semiconductive polyolefin composition according to claim 1, wherein the polar co-monomer is selected from the group consisting of acrylate and acetate co-monomers.
 11. The semiconductive polyolefin composition according to claim 1, wherein the silane-group containing co-monomer is selected from the group consisting of vinyl tri-alkoxy silanes.
 12. The semiconductive polyolefin composition according to claim 1, wherein the polymer (II) is selected from the group consisting of polyethylenes, polypropylenes or polybutylenes.
 13. The semiconductive polyolefin composition according to claim 12, wherein the polymer (II) is a homopolymer, a random copolymer or a heterophasic copolymer of propylene.
 14. A power cable comprising at least one semiconductive layer comprising a semiconductive polyolefin composition according to claim
 1. 15. The power cable according to claim 14, which is a medium voltage power cable.
 16. A power cable comprising a conductor, a first semi-conductive layer (a), an insulation layer (b) and a second semiconductive layer (c) each coated on the conductor in this order, wherein at least one of the first and second semiconductive layer(s) ((a) and (c)) comprises a semiconductive polyolefin composition according to claim
 1. 17. The power cable according to claim 16, wherein the first (a) and the second (c) semiconductive layer is cross-linked.
 18. The power cable according to claim 16, wherein the second semiconductive layer (c) is non-strippable from the insulation layer.
 19. The power cable according to claim 16, which is a medium voltage power cable.
 20. A process for producing a power cable comprising a conductor, a first semiconductive layer (a), an insulation layer (b) and a second semiconductive layer (c) each coated on the conductor in this order, comprising the steps of: (i) providing the semiconductive polyolefin composition according to claim 1, by blending the polyolefin (I), carbon black and, optionally, the polymer (II) together, optionally with additives, at a temperature above melting point of the polymer components, (ii) co-extruding the melt at elevated temperature above the melting point of the polymer components, together with the polymer melt for any of the other layers, on a conductor for forming a layered power cable structure, and (iii) winding the obtained layered power cable onto a cable drum.
 21. The process according to claim 20, wherein the power cable comprises at least one cross-linkable layer, a cross-linkable insulation layer (b) and a cross-linkable second semiconductive layer (c).
 22. The process according to claim 20, wherein the power cable is further subjected to a cross-linking step at elevated temperature.
 23. The process according to claim 20, wherein the cross-linking step includes a steam or water treatment.
 24. The process according to claim 22, wherein the cross-linking step is effected at a cross-linking temperature of not less than 75° C.
 25. The process according to claim 20, wherein the power cable comprises at least a cross-linkable first semiconductive layer (a), a cross-linkable insulation layer (b) and a cross-linkable second semiconductive layer (c). 